1. Field of the Invention
The invention concerns blown microfiber (BMF) webs of polyolefin and surfactant that are hydrophilic and so can be used as wipes and in other liquid-absorbing applications. The invention also is concerned with a method for making such BMF webs and apparatus by which they can be made.
2. Description of the Related Art
BMF webs are made by extruding a thermoplastic polymer or resin such as a polyolefin through a row of small, side-by-side orifices into a high velocity gaseous stream which attenuates the emerging resin into microfibers. The gaseous stream creates a turbulence which entangles the microfibers to form a coherent web that is deposited onto a collector such as a moving screen. For uses requiring good web strength, the web may be calendered between a heated steel roll and a patterned roll, even though this lessens the capacity of liquids that it can absorb and retain.
One method of making the BMF web hydrophilic is to incorporate a surfactant into the web by means of extruding a thermoplastic resin/surfactant premix. When the thermoplastic resin is polypropylene, an amount of surfactant exceeding 6% by weight of the resin may be required to maximize the hydrophilicity and absorbancy properties of the web. More commonly, surfactant is topically applied, e.g., by being sprayed onto a finished BMF web. See U.S. Pat. No. 31,885 (Meitner) which concerns the use of BMF webs as wipers or wipes that can clean off both water and oil from a surface in a single pass.
By being applied from a spray, much less surfactant is required to make a BMF web hydrophilic than when the surfactant is added to the resin before the microfibers are blown. However, it is more difficult to attain uniform surfactant treatment by spraying and, in areas receiving excess application levels, web blockage can occur. Furthermore, spraying of surfactant is messy and requires the worker who does so to wear a respirator.
The predominance of topically applying surfactant onto a finished BMF web is highlighted by a statement in U.S. Pat. No. 4,578,414 (Sawyer et al.), the invention of which "differs from the prior art by incorporating surface active agents directly into the bulk polymer resin rather than introducing a copolymer or applying a surface treatment to fabricated fibrous structures" (col. 6, 1s. 23-26). Sawyer goes on to say that preferred blends comprise about 95% to about 99.9% of the olefin polymer, the remainder being the surfactant.
FRD Offenlengungsschrift P 25 43 781 (Grammer) which was laid open Apr. 15, 1976, concerns a battery separator wall or spacer made from a BMF web of a mixture of thermoplastic resin and surfactant. The thermoplastic resin may be polyolefin, such as polyethylene and polypropylene, or polystyrene, or a copolymer of propylene with another monomer. The Offenlegungsschrift alleges two discoveries about the BMF webs. First, it says that if the surfactant is nonionic, the process of extruding the BMF can be performed at a lower temperature than had previously been possible. Second, it says that a higher extrusion throughput rate can be achieved when the surfactant is mixed with the thermoplastic resin while it is being conveyed through the extruder instead of being pre-mixed with the resin. The Offenlegungsschrift imagines the extruder as having five zones of equal length and preferably adds the surfactant at the third or middle zone. In its examples, from 2.5 to 4.5% of nonionic surfactant such as an ethoxylated addition product of propylene oxide with propylene glycol or dodecylphenyl poly(ethylene glycol) was used. Except for use as a battery separator or spacer, the Offenlegungsschrift suggests no utility for its web.
U.S Pat. No. 4,070,218 (Weber) concerns a nonwoven web which differs from a BMF web in that its fibers are substantially continuous and, as deposited, are substantially not fused together at crossing points. The Weber web is made from a mixture of a thermoplastic resin such as polypropylene and a surfactant (there called a "lubricating agent"). "Alternatively, the lubricating agent may be metered directly into extruder 12 if desired" (col. 2, 1s. 57-59). Then after being calendered, "the bonded web 34 is heated to cause the lubricating agent to 15 migrate to the fiber surfaces" (col. 4, 1s. 1-4). Weber's surfactant has a molecular weight in the range of from 200 to 4000.